1. Field of the Invention
The present invention relates to a video encoder and a portable radio terminal device using the video encoder.
2. Description of the Related Art
An MPEG system, which is a typical technique for compressively coding motion pictures, carries out coding by combining inter-prediction (motion compensation prediction), intra-prediction, discrete cosine transformation (DCT), and variable length coding. With the intra-prediction, pixel values for a block to be coded are predicted on the basis of pixel values for surrounding reference blocks. A predictive residual is coded which is the difference between each predicted value and the corresponding actual pixel value.
On the other hand, a motion picture coding standard called H. 264/AVC has been established by improving the MPEG system. With H. 264/AVC, for complicated images, intra-prediction is carried out for each block composed of 4×4 pixels (this is called 4×4 intra-prediction). For flat images, intra-prediction is carried out for each macro-block composed of 16×16 pixels (this is called 16×16 intra-prediction). This improves coding efficiency. With the 4×4 intra-prediction, one of nine prediction modes, modes 0 to 8, is selected and determined for each block.
The following technique is disclosed in “Study on Improvement of Efficiency of Intra-Prediction System in H. 264 Coding”, KDDI Research Institute, Koichi TAKAGI, et al., ITE Winter Annual Convention 2004): a technique for calculating an evaluative value corresponding to a 4×4 intra-prediction mode in a process for determining a coding prediction mode for each macro-block. With this technique, an original image signal, that is, a motion picture to be coded, is used as a reference image signal to determine the evaluative value. Then, an optimum coding prediction mode is determined on the basis of the evaluative value. Moreover, Koichi TAKAGI, et al. discusses the effects of the amount of codes representing modes at a low rate which effects are observed if a mode evaluative value for a rate distortion optimization (RDO):off mode is used. The following method is disclosed in “Joint Model Reference Encoding Methods and Decoding Concealment Methods JVT-J049 2004-03-02” a method of using the sum of absolute differences (SAD) relating to predictive residuals or the sum of absolute transformed differences (SATD) obtained by subjecting predictive residuals to an Hadamard transformation in order to determine an evaluative value for a prediction mode in H. 264/AVC.
Koichi TAKAGI, et al. uses the original image signal as a reference image signal to determine an evaluative value for a 4×4 intra-prediction mode. This drastically reduces the amount of calculation required to determine a coding prediction mode for each macro-block, compared to a conventional technique of using a local decoding image signal as a reference image signal. However, depending on the position of the 4×4-pixel blocks, there may be large differences in predictive residuals between the use of the original image signal as a reference image signal and the use of the local decoding image signal as a reference image signal. Consequently, the correct evaluative value is not always obtained, thus precluding an appropriate prediction mode from being determined.